Transition metal dichalcogenide, TMDC, materials have attracted large interest due to their properties. TMDC materials are typically thin semiconductors of the type MX2, with M a transition metal atom and X a chalcogen atom. In the TMDC materials one layer of M atoms is sandwiched between two layers of X atoms typically forming a monolayer sandwich having a sub 10 Å thickness. TMDC crystals are formed of monolayers of the above kind, which bound to each other by van der Waals attraction. TMDC monolayers have properties that attract interest in several fields of technology.
For instance, TMDC monolayers having specific combinations of M and X exhibit a direct band gap making such materials highly interesting and relevant in various types of electronics, such as transistors, optical emitters, and optical detectors.
Typically, the electron mobilities of TMDC monolayers are higher than the ones of silicon, making TMDC monolayers usable throughout a wide range of electronics. Moreover, the strong spin-orbit coupling in TMDC monolayers allows control of the electron spin. Furthermore, TMDC monolayers are structurally stable.
Several techniques for forming or fabricating TMDC materials or TMDS monolayers exist.
Exfoliation may be used for producing small size samples of TMDC materials. By exfoliation of TMDC monolayers from a bulk material, small samples having sizes typically in the range of 5-10 μm may be produced. Exfoliation, however, may not be feasible for producing larger samples and samples having a well-controlled structure, due to the stochastic nature of the exfoliation process. In other words, samples produced by exfoliation are generally difficult to use for device fabrication in practice.
Other alternatives for producing TMDC materials include Sulfurization, Atomic Layer Deposition, ALD, Chemical Vapor Deposition, CVD, and solid source Molecular Beam Epitaxy, MBE.
When forming TMDC materials by a sulfurization process, the sulfur precursors used must be cracked in order to be able to combine and form the desired material. The cracking of the precursor in question is typically used by heating the precursors to a sufficient elevated temperature. Further, the sulfurization process itself generally requires elevated temperatures at the substrate which may result in damages to the substrates and potentially any structures formed prior to the sulfurization process.
In ALD single atom layers may be grown on top of each other thereby enabling the formation of a layered structure. However, when forming a TMDC material by ALD, the TMDC material will have to undergo a high temperature anneal process in order to satisfy reasonable quality requirements.
Further, in CVD reactants that are to form the TMDC material are vaporized and delivered to a substrate at high temperatures, typically above 600° C. The reactants are then allowed to react at the substrate, thereby forming the TMDC material.
The high temperatures needed in ALD and CVD processes risk damaging the substrates used and potentially any structures formed prior to the processes.
Another approach is to use solid source MBE where pure M and X are evaporated and delivered to a substrate by means of effusion cells. When using MBE for fabrication of TMDC materials, the high vapor pressure of the X material brings about problems associated with undesired vaporization of the X material resulting in tool contamination already at moderate temperatures, thereby rendering the use of solid source MBE less attractive.
Although conventional techniques for fabricating TMDC materials are available, challenges remain to be overcome the above described drawbacks related to non-uniformity, sample size, temperature, and contamination, before large-scale fabrication of TMDC materials become feasible. Hence, there is a need for an improved method for fabrication or formation of a transition metal dichalcogenide, TMDC, material layer.